1920×1200 HiDPI on my 15.4″
At first it seemed too small and I used 1680×1050 a lot, but lately Ive found 1920 to be perfect. I sometimes use 2880×1800 when I want pixel-perfect accuracy and lots of space.
Im not sure if youre referring to the built-in more space option, or simply commenting that you like to have more space.
But for the sake of clarity to other reading this, Ill mention that on the 15.4″ rMBP the more space setting yields a 1920×1200 HiDPI resolution. As others have said above, there is no built-in option for true native 2880×1800: you have to use a third-party program such as RDM, QuickRes, or SwitchResX.